Heat, a form of kinetic energy, is generally transferred in three ways: conduction, convection and radiation. The transfer of heat energy by radiation makes possible the transfer of large amounts of heat from the sun to the earth. Radiation transfers the sun's heat to exposed surfaces, such as the roof or wall of a building or parked cars.
When the air inside a building is cooler than the roof and outside air, conduction will carry heat through the roof where it is distributed into an attic space or other interior areas near the roof by radiation and convection. Similarly, heat can also be transferred through the exterior walls of a building to its interior. Heat may also be conducted away from the interior of a building through the roof and other exterior surfaces to the outside air if the interior air is warmer.
Several ways of reducing roof temperatures as well as blocking heat from entering windows have been developed and put into practice. Some of these include the application of water spray or mist alone to rooftops, the application of white, reflective coatings to a roof surface (the “cool roof” technology), and even placing containers of plants on a roof. A few types of shading panels have been developed to shade air conditioning units, but they are typically constructed of metal and/or fiberglass, and often stay hot due to the reradiation of heat on those panels. The reradiated heat, if not removed, is transferred into the building.
Parking lots have also been covered to block sun and/or rain. Such coverings, if water-repellant, required specifically designed support structures. Also, some coverings use materials that are not efficient at blocking or reflecting solar radiation. Such covers may still allow a parking lot to become quite warm as the covers heat up and transfer the heat by conduction to underlying air.
Insulation materials have been improved over the years and adding extra insulation underneath rooftops or walls can help prevent heat from flowing through a building's roof and walls. However, insulation merely retards heat flow into the building from the exterior surfaces rather than preventing or reducing the heating of the exterior surfaces. Insulation is generally not used in parking lot covers at all.
Prevention is a key strategy in types of energy saving devices and products such as cool roof coatings and exterior solar screens installed on the outside of window frames. Austin Energy stated in a press release dated Jun. 7, 2000, under “Biggest Money Saving Improvements”, that one of the four “key home energy improvements that provide the greatest energy savings” is the installation of solar screens on the outside of windows that are hit by direct sunlight. Such screens prevent 60%-70% of sunlight/heat that would normally pass through the windows from entering the house. However, this technology affects only direct heat radiation entering a building through windows and fails to address the large amounts of heat absorbed by all exterior surfaces of a building.
In the case of cool roof coatings, they are mostly applied to flat or low-sloped rooftops in warm climate zones. The objective is to provide and maintain a high level of reflectivity along with a high degree of infrared emissivity so that heat “build up” on the rooftop is prevented by the turning away the sun's light and heat. These technologies can often be thwarted by peeling or cracking or other deterioration of the coating as well as by the accumulation of dirt on the coating which is difficult to remove and which reduces the performance of the cool roof technology. These and other problems are discussed in Kim, W. A., Rohm & Haas Co., “Selective Quality Coatings—Not All Roof Coatings Are Created Equal”, —at http://www.energy-seal.com/esweb.nsf/newsroom/Selecting-Quality-Coatings, accessed Jan. 22, 2002. See also Robin Suttel, “Roofing in a Greener World” Buildings.com, (accessed Oct. 25, 2002 at http://www.buildings.com/Articles/details.asp?ArticleID=1063).